Waste heat won't make membrane distillation cool: Thermodynamic analysis of cooling and pumping burdens

Membrane distillation is promoted as a thermally driven desalination process capable of utilizing low-grade heat, yet its full thermal and hydraulic burdens have not been comprehensively resolved. The cooling burden is often neglected altogether, an assumption rarely met in real applications and especially in severe water stressed regions. This study develops a lumped thermodynamic framework that quantifies heating, cooling, and pumping duties across three representative membrane distillation configurations. Results show that cooling loads can reach the same order of magnitude as heating, up to 80–100% of the thermal input in open-loop feed setups, and remain of comparable magnitude even with internal or external heat recovery. Pumping penalties of 0.2–0.5 kWh/m3 emerge at typical single-pass water recoveries of only 2–6% and pressure losses of 300 mbar, underscoring the need for joint thermal–hydraulic optimization. The analysis also suggests a techno-economic trade-off: lowering specific energy consumption requires efficient heat utilization, typically achieved through effective system-level integration, large modules, minimization of terminal temperature differences, and internal or external heat recovery solutions; whereas compact designs entail modest capital expenditures but disproportionately high operational expenditures. Finally, a methodology for converting classical energy requirement indicators into actual energy consumption values is presented. The analysis compares different heating and cooling strategies, showing that feasibility relies not only on “free” heat availability, but also on heat sinks effectiveness, optimized heat recovery, and low-resistance module hydraulics.